Method and arrangement for venting a tank

ABSTRACT

A method for alternately carrying out phases with and without tank venting during operation of an internal combustion engine equipped with a tank-venting assembly is characterized in that the ratio of the time spans with and without tank venting is selected to be dependent upon operating data of the engine or of the tank-venting assembly. Preferably, a variable is measured which is a measure for the fuel quantity to be regenerated during tank venting and the above-mentioned ratio is increased in favor of the tank-venting time span with respect to the base ratio when the value of the measured variable exceeds an upper limit (Dp --  SMW; FTEA --  SWU). This method makes possible that an adsorption filter and a tank-venting valve in the corresponding arrangement can be dimensioned for lesser throughput quantities than previously without the danger being present that fuel vapors escape to the ambient. The tank-venting time span is extended with respect to the base-adaptation time span when a large amount of fuel vapor occurs whereby the smaller adsorption filter still regenerates satisfactorily notwithstanding the reduced cross section of the tank-venting valve.

FIELD OF THE INVENTION

The invention relates to a method and an arrangement for alternatelycarrying out phases with and without tank venting during operation of aninternal combustion engine equipped with a tank-venting assembly.

BACKGROUND OF THE INVENTION

U.S. Pat. No. 4,705,007 describes a method according to which phaseswith and without tank venting (namely, tank-venting phases andbase-adaptation phases) alternate in a fixed raster. 5 minutes areprovided for the tank-venting time span and 1 minute is provided for thebase-adaptation time span. In practice, the first time duration is morelikely to be somewhat shorter and the second somewhat longer.

The duration of the tank-venting time span together with thecharacteristic variables of the tank assembly and of the correspondingengine determine the size of the adsorption filter in which fuel vaporis adsorbed from the tank. These variables also determine the diameterof the tank-venting valve with the aid of which the adsorption filter ispurged with air. The size of the adsorption filter and the cross sectionof the tank-venting valve must be so dimensioned that, even for thelargest possibly occurring fuel vapor quantity, essentially all fuelvapor can be adsorbed during the base-adaptation time spans and canagain be desorbed during the tank-venting time spans.

In the technology, the problem generally is present to operatearrangements according to such methods and to so configure thearrangements that the components are used in the most purposeful mannerpossible. This problem applied correspondingly also to methods andarrangements for carrying out phases with and without tank ventingduring operation of an internal combustion engine with a tank-ventingassembly.

SUMMARY OF THE INVENTION

A method of the invention of this kind is characterized in that theratio of the time spans with and without tank venting is no longerfixed; instead, the ratio is selected to be dependent upon operatingdata of the engine or of the tank-venting assembly.

The arrangement of the invention includes a sequence control foralternately carrying out phases with and without tank venting. Thesequence control is so configured that it selects the ratio of phasedurations in dependence upon operating data of the engine or of thetank-venting assembly.

The method and arrangement can use participating components with greaterflexibility than was previously possible because this method and thisarrangement no longer use a fixed pregiven time reference for theabove-mentioned phases.

In a preferred embodiment, the method includes the following steps:

a variable is measured which is a measure for the fuel quantityoccurring during the tank venting; and,

the ratio of the tank-venting time span to the base-adaptation time spanis increased with respect to a base ratio when the value of the measuredvariable exceeds an upper threshold value.

According to another variation, tank venting is carried out at full loadwithout lambda control always with a completely open tank-venting valve.This variation is based upon the recognition that no base adaptation canbe carried out in the phases without tank venting during full loadwithout lambda control so that it is more purposeful to utilize theentire time for tank venting. The valve is not used much because thevalve is held continuously open in lieu of being clocked.

According to a third variation, a diagnostic method is started todetermine the operability of the tank-venting assembly during atank-venting phase. The diagnostic method requires a temporary closureof the tank-venting valve. Immediately with the closing of the valve, abase-adaptation phase is started and the next tank-venting phase isextended at least partially as compensation for the interrupted previousphase. In this way, the time for diagnosis is at the same timepurposefully utilized for adaptation.

It is especially advantageous to utilize all above-mentioned variationsin common.

The method having a variable ratio of the above-mentioned time spansmakes possible that the adsorption filter and the tank-venting valve canbe configured for the throughput of an average quantity of fuel from thetank venting in lieu of a maximum quantity. These parts are configuredto be smaller than previously but are nonetheless capable tosatisfactorily vent even very large quantities of fuel vapor as theyoccasionally occur because, in this case, the tank-venting time span isextended at the expense of the base-adaptation time span. The shorteningof the base adaptation time span, for example, up to 1 minute, and theextension of the spacing between two such time spans, for example, to 15minutes (duration of the extended tank-venting time span) leads only inexceptional cases to disadvantages, for example, for a very rapid uphilltrip on a relatively steeply inclining roadway. Here, it could actuallybe necessary that the factor considering air density should change by 5%or more in the base adaptation in the above-mentioned time span. Sincethe factor cannot do this because of the blocked base adaptation, therequired change in the fuel injection time spans must be taken up by thecontrol output of the lambda control which, in principle, is possiblewithout difficulty since the typical range of the lambda control amountsto approximately 15%. Difficulties occur for a short time duringtransient operations for changes between greatly different operatingconditions because then only the relatively sluggish control mustundertake the adaptation to the new operating condition without optimalsupport by means of a well adapted precontrol. Apparently, thedisadvantage is present that short-term increases of the exhaust-gasoutput can occur during transient operations when there is an extremelyhigh occurrence of fuel vapor in the tank venting and therewith greatlyextended tank-venting time spans during the time that a very rapid andsteep uphill drive takes place. All these conditions are, however,satisfied only very infrequently. However, the advantage is continuouslypresent that a smaller adsorption filter and a smaller tank-ventingvalve can be used. This leads to permanent savings of fuel if only avery small savings of fuel because of the reduced weight of these partsand also a reduced output of exhaust gas. Furthermore, the energyconsumption for producing and operating the parts is reduced.Accordingly, substantially greater advantages are obtained compared tothe above-mentioned disadvantage which occurs only infrequently.

The quantity of fuel vapor occurring during tank venting couldtheoretically be most precisely detected by means of a through-flowsensor between the tank and the adsorption filter. Such a through-flowsensor would be, however, most expensive and complex when it is tofunction precisely. It is simpler to determine the pressure differencebetween the tank pressure and the ambient pressure. For this purpose, apressure-difference sensor is required on the tank which is recommendedfor mounting for several reasons for modern tank-venting assemblies. Thepressure-difference sensor is therefore often provided for otherreasons. The greater the pressure difference measured by the sensor, themore intense the fuel in the tank vaporizes. The ratio of thetank-venting time span to the base-adaptation time span can therefore bemade dependent on this pressure difference. Another very advantageouspossibility is that the above-mentioned ratio can be made dependent fromthe tank-venting adaptation factor itself. This is namely a directmeasure for the fuel vapor quantity occurring instantaneously duringtank venting. However, this value is not made actual during thebase-adaptation time span.

It is advantageous to extend the base-adaptation time spans at theexpense of the tank-venting time spans when only little fuel vaporoccurs during tank venting. In the last-mentioned time spans, thetank-venting valve is clock-driven; whereas, this valve is closedwithout current in the base-adaptation time spans. The valve thereforecontributes significantly to the increase of the service life of thetank-venting valve when this valve is then only driven when actuallyrequired for tank venting. Another type of drive of reduced usementioned above is that the valve is held continuously open which ispossible at full load without lambda control.

The answer to the question as to how much the tank-venting time spanshould be extended in order to prevent an oversaturation of theadsorption filter is dependent not only upon how much fuel vapor issupplied to the filter from the tank, but also on how well the filtercan be purged in a particular operating state. In idle, and at lowloads, a pressure at the output of the tank-venting system is so lowthat the quantity of purging gas must be limited by a partial closure ofthe tank-venting valve (corresponding to the pulse-duty factor). Incontrast, at very high load, and especially at full load, the purgingeffect is somewhat low even for a completely opened tank-venting valve.It is therefore advantageous to not only increase the tank-venting timespan for increasing quantities of fuel vapor supplied to the adsorptionfilter but also during increasing load, that is, reduced purging action.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be explained with reference to the drawingswherein:

FIG. 1 is a block diagram of an internal combustion engine having afuel-venting assembly and lambda control as well as function groups fortank-venting adaptation and base adaptation;

FIG. 2 is a flowchart for explaining a procedure to increase thetank-venting time span at the expense of the base-adaptation time spanbased on a difference-pressure signal;

FIG. 3 is a flowchart corresponding to that of FIG. 2 but for anadditional reduction of the ratio between tank-venting time span andbase-adaptation time span with the change of the ratio taking place onthe basis of the tank-venting adaptation factor;

FIG. 4 is a flowchart for explaining the method for alternately carryingout the base adaptation and the tank-venting adaptation;

FIG. 5 is a flowchart for explaining a method for exclusively carryingout tank venting at full load;

FIG. 6 is a flowchart for explaining a method for starting the baseadaptation directly with closure of the tank-venting valve during atank-venting phase for diagnostic purposes; and,

FIG. 7 is a flowchart for explaining a method for starting thetank-venting phase after transient oscillations of the base adaptation.

DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

FIG. 1 shows an internal combustion engine 10 having an intake pipe 11in which a throttle flap 12 and an injection valve 13 are mounted and anexhaust gas pipe 14 in which a lambda probe 15 is arranged. Theinjection times with which the injection valve 13 is driven aredetermined by adapted precontrol with lambda control. For this purpose,injection times are read out of an injection-time characteristic field16 in dependence upon rotational speed n and load L and are logicallycombined with adaptation variables and a control factor FR. The controlfactor FR is made available by a lambda controller 17 which forms thisfactor on the basis of a control algorithm starting from a controldeviation as this deviation corresponds to the difference between alambda desired value read out of a desired value characteristic field 18and the lambda actual value supplied by the lambda probe 15. The controlfactor FR, that is the control output of the lambda control, is thebasis for the adapted values as they are formed by a base-adaptationunit 19 and a tank-venting adaptation unit 20. The base-adaptation unit19 here computes various corrective variables in any desired knownmanner. In FIG. 1, three variables, which are not described in greaterdetail, are shown for the base adaptation. Here, the first variable canadapt additive leakage-air defects, the second variable can compensatefor multiplicative changes in air tightness and the third variable canadapt additive pull-in time and release time changes of the injectionvalve 13. The tank-venting adaptation unit 20 makes available: amultiplicatively operating factor FTEA for the tank venting which hasthe value one during a non-operating tank venting, and, in contrast, inthe case of an operating tank venting, has an adaptive value greater orless than one in dependence upon whether the tank venting supplies aleaner or a richer mixture to the intake pipe than is provided duringthe mixture formation without tank-venting adaptation.

As mentioned above, fuel can be supplied to the internal combustionengine 10 in two ways, namely, either via the injection valve 13 or viaa venting line 21 of a tank-venting assembly. The injection valve 13receives its fuel via a fuel pump 22 from a tank 23. This tank 23 isvented via an adsorption filter 24, a tank-venting valve 25 and theventing line 21. As long as the tank-venting valve 25 is closed, fuelvapor emanating from the tank 23 collects in the adsorption filter 24.Base adaptation takes place during this time. The tank-ventingadaptation unit 20 receives the value one as the input value which hasas a consequence that no adaptation is carried out. The tank-ventingadaptation unit 20 emits the value one as a tank-venting factor FTEA.

As soon as the tank-venting valve 25 is opened, the underpressurepresent in the venting line 21 acts in the adsorption filter 24whereupon this filter draws purging air through a venting line 26. Thisair desorbs fuel held in the adsorption filter and conducts the same tothe intake pipe 11. Tank-venting adaptation is undertaken in this phase.For this purpose, the tank-venting adaptation unit 20 receives theoutput signal FR from the lambda controller and emits the tank-ventingadaptation factor FTEA. The base-adaptation unit 19 receives the valueone as input value during this tank-venting time span. In this way, thebase adaptation variables remain unchanged which continue to be emittedcorresponding to their last state.

The tank-venting valve 25 is not necessarily completely open in thetank-venting time spans. Rather, it is, as a rule, driven with aspecific pulse-duty factor which is read out of a pulse-duty factorcharacteristic field 27 in dependence upon engine speed n and load L.The pulse-duty factors are so dimensioned that a maximum air quantitycan pass through the tank-venting valve 25. At idle, this quantity isrelatively greatly limited; whereas, at full load, the tank-ventingvalve is completely opened. When the adsorption filter 24 is completelyregenerated, the pulse-duty factor TVH, which is read out of thepulse-duty factor characteristic field 27, remains unchanged. Otherwise,the pulse-duty factor TVH would be reduced with the aid of a limit-valuecontrol 28 in dependence upon the value of the tank-venting factor FTEA.The limit-value control emits a factor FTVH which maximally assumes thevalue one. The richer the mixture sup,plied from the tank-venting line21 into the intake pipe 11 is, the more the pulse-duty factor TVH, whichis read out of the pulse-duty factor characteristic field 27, is reducedwith the aid of the above-mentioned factor FTVH.

The switchover between base adaptation GA and tank-venting adaptationTEA takes place with the aid of a sequence control 29.

The arrangement described to this extent corresponds completely with anembodiment of a conventional arrangement. The difference is in thespecific configuration of the sequence control 29. In known methods andarrangements, the sequence control is based on fixed values for thebase-adaptation time span and the tank-venting time span for alternatelycarrying out base adaptation GA and tank-venting adaptation TEA. Thebase-adaptation time span and the tank-venting time span are typically1.5 minutes and 4 minutes, respectively. However, with the invention,the sequence control 29 varies the ratio of tank-venting time span tobase-adaptation time span in dependence upon the fuel quantity occurringduring the tank venting.

A direct measure for the fuel vapor quantity occurring during tankventing is the value of the tank-venting adaptation factor FTEA. Whenthis value indicates a very rich tank-venting mixture, the tank-ventingtime span is extended and the base-adaptation time span is shortened. Anopposite change of the above-mentioned time spans takes place in theopposite case. It should however be noted that, when selecting thevariable FTEA as a measure for the fuel quantity occurring during tankventing, the base-adaptation time span cannot be selected to be too longsince the variable FTEA is not actualized in this time and therefore itis unknown whether too much or too little fuel has collected in theadsorption filter 24.

Very large base-adaptation time spans can however be selected when as ameasure for the fuel quantity to be regenerated, the pressure differencebetween the internal pressure of the tank 23 and the atmosphericpressure is used. For this purpose, a difference pressure sensor 30 isconnected to the tank. The signal of the sensor 30 is supplied to thesequence control 29. The difference pressure is a direct indication asto whether more or less fuel has vaporized and accordingly is anindication as to how much fuel is to be regenerated. If the pressuredifference at first was very low and therefore a long base-adaptationtime span had been selected, and nonetheless an increase of the pressuredifference is observed during this time span, the base adaptation can beinterrupted and tank venting can be carried out.

It will now be described with respect to FIG. 2 as to how thebase-adaptation time span T₋₋ GA and the tank-venting time span T₋₋ TEAcan be selected in dependence upon values of the difference pressure Dp.In a step s2.1, a check is first made as to whether Dp is less than alower threshold value Dp₋₋ SWU. If this is the case, then in step s2.2,an extended base-adaptation time span of 10 minutes and a usualtank-venting time span of 4 minutes is set. Otherwise, an inquiry ismade in step s2.3 as to whether Dp is less than a mean threshold valueDp₋₋ SWM. If this is the case, then conventional time spans are selectedas they are shown in a step s2.4 in FIG. 2. Otherwise, an inquiry ismade in step s2.5 as to whether the difference pressure Dp is below ahigh threshold value Dp₋₋ SWH. If this is the case, then in step s2.6,the base-adaptation time span is shortened to 1 minute anti thetank-venting time span is extended to 6 minutes. Otherwise, that is forvery high difference pressure, the tank-venting time span is lengthenedstill further in a step s2.7, namely, to 15 minutes. The base-adaptationtime span however remains at 1 minute. In the embodiment, this is theshortest time span within which the base adaptation can still bepurposefully carried out.

FIG. 3 shows a similar procedure when, in lieu of the pressuredifference Dp, the tank-venting adaptation factor FTEA is used as ameasure for the quantity of fuel to be regenerated during the tankventing. Differences are that in the last case the base-adaptation timespan must not be extended for a reason given above and that theabove-mentioned factor is reduced with increasingly greater fuelquantity while the pressure difference in this case is greater. Thisleads to changed inquiries.

In a step s3.1, a check is made as to whether the value of FTEA is lessthan a lower threshhold FTEA₋₋ SWU. If this is the case, then thebase-adaptation time span is shortened to the minimum value of 1 minutein a step s3.2 and the tank-venting time span is extended to 10 minutes.Otherwise, in step s3.3, an inquiry is made as to whether the value ofFTEA lies bellow a high threshold FTEA₋₋ SWH. If this is the case, thenin step s3.4, the usual time spans are set which define the base ratioof the tank-venting time span to the base-adaptation time span.Otherwise, in step s3.5, the tank-venting time span is shortened to 3minutes whereas the base-adaptation time span is increased slightly to 2minutes. A larger extension is not acceptable since the value FTEA isnot actualized during the base-adaptation phases and it is thereforeunclear as to whether the fuel quantity to be regenerated has changed.

It is especially advantageous to combine the two procedures describedwith respect to FIGS. 2 and 3 in that the ratio of the tank-venting timespan to the base-adaptation time span is actually set with the aid ofthe precise value FTEA; that, however, in the extended base-adaptationtime spans, a check is made with the aid of the difference pressure Dpas to whether the base adaptation should be interrupted because ofincreasing vaporized fuel quantity.

FIG. 4 shows how the change of base-adaptation phases and tank-ventingphases can be controlled. In a step s4.1, the base adaptation is firststarted after running through two marks A and B (see also FIG. 5 forthis purpose). In a next step s4.2, an inquiry is made as to whetherbase adaptation is just then taking place. Since this is the case afterthe start of the method, a check is made as to whether thebase-adaptation time span T₋₋ GA has already run (step s4.3). Theinformation for the actual time span T₋₋ GA is supplied from a block b1.This time span has not yet elapsed shortly after the start of the methodwhereupon a step s4.8 follows step s4.3. In step s4.8, the inquiry ismade as to whether the method should be ended. This is not yet the casewhereupon the sequence repeats starting with step s4.2. If after a timeit is determined in step s4.3 that the actual value of thebase-adaptation time span T₋₋ GA is reached then, in step s4.5, the baseadaptation GA is ended and the tank-venting adaptation TEA is started.Thereafter, a check is made (step s4.6) as to whether the actualtank-venting time span T₋₋ TEA has already run. The value of this timespan is made available from a block b2. If the time has not yet elapsed,the steps s4.8, s4.2 and s4.6 repeat after running through two marks Cand D (see also FIG. 6 for this purpose). This run-through is continueduntil the time span T₋₋ TEA has elapsed. Then the tank-ventingadaptation is ended and the base adaptation is again started (steps4.7). If necessary, the described sequence starting with step s4.2 isagain repeated after step s4.8 of the inquiry of the end of the method.

The current values of T₋₋ GA and T₋₋ TEA as they are read out of blocksb1 and b2, respectively, are determined in accordance with one of themethods explained with respect to FIGS. 2 and 3. For the time span T₋₋TEA, there is indicated in parentheses in block b2 that this variablecan be selected so as to be in addition dependent upon load. Thisconsiders the fact that, at high loads, only a slight pressuredifference exists between venting line 21 and ventilating line 26 at theadsorption filter 24 so that the filter is only slightly regenerated. Itis now assumed that a constant difference pressure is measured by thedifference pressure sensor 30. The fuel vapor quantity occurring at thismean difference pressure can be better regenerated at average loads thanat high loads. It is therefore advantageous that the ratio of thetank-venting time span to the base-adaptation time span is not onlyselected to be dependent upon difference pressure Dp but also dependentupon engine speed n and load L. The load condition is however of lesssignificance when the above ratio is set with the aid of thetank-venting adaptation factor FTEA. If at first at higher loads onlytoo little is regenerated, then this leads to a reduction of the factorFTEA which results automatically in an extension of the tank-ventingtime span.

it is noted that there are many strategies for base adaptation andtank-venting adaptation. What is important in the method described aboveand the arrangement described above is however completely independent ofthe particular adaptation method. What is alone decisive is that thetime spans for the adaptations, irrespective of how they are executed,are dependent on the value of a variable which is a measure of the fuelquantity to be regenerated during tank venting and which time spans canbe dependent furthermore upon the load state of the internal combustionengine provided with adaptation.

FIG. 5 shows an embodiment as it can be used independently or alsobetween the marks A and B in the sequence of FIG. 4. A check is made asto whether full load is present (step s5.1). If this is the case, thentank venting is carried out (step s5.2) and step s5.1 is repeated untilthe result is there obtained that the inquired condition is no longersatisfied. This procedure is based on the recognition that at full loadfor engines having lambda control, this lambda control is generallyswitched off and for this reason, no base adaptation can be carried out.Accordingly, it is not purposeful to interrupt the tank venting which atfull load in any event does not operate too effectively.

FIG. 6 shows an embodiment as it can be used independently or alsobetween the marks C and D in the sequence of FIG. 4. A check is made(step s6.3) as to whether a tank assembly diagnosis should be carriedout for a closed tank-venting valve. A method of this kind is describedin a parallel application. According to this method, the tank-ventingvalve is closed after a buildup of underpressure at the adsorptionfilter in order to obtain a conclusion as to the operability of theassembly from the time trace of the decay of the underpressure whichthen results. The closure of the valve and the diagnosis are subjectmatter of step s6.2 in FIG. 6. The tank-venting phase is ended with theclosure of the valve and an adaptation phase is started and anamplification factor for the next tank-venting time span is emitted(step s6.3). The advantage of this measure has already been presentedabove. The amplification factor has the value two in the embodiment. Fora common application with obtaining a reference quantity according tothe sequence of FIG. 3, it is purposeful to limit the maximumtank-venting time span as it is obtained by multiplication by theamplification factor for the reasons explained in connection with FIG.3.

FIG. 7 shows an embodiment wherein, after the start of the internalcombustion engine (combustion engine) a delay is first had until thetransients have subsided. If this is the case, then the tank-ventingvalve is continuously opened.

For this purpose, an inquiry is made in a step s7.1 after the start ofthe engine as to whether the base adaptation (GA) is active. A conditionprecedent therefor is for example the operational readiness of thelambda control. Only with active base adaptation, a step s7.2 follows,in which the value of the base-adaptation variable GAG is intermediatelystored as value GAGm with said value GAG being just then current. Thestep s7.3 operates to reset a timer to the value zero. In the stepsequence s7.4, s7.5 which follows, the value of the variable timerincreases until in s7.5, the threshold value Ta is exceeded. At thistime point, the current value GAG of the base adaptation variable iscompared to the intermediately stored value GAGm in the step s7.6. Ifthe difference of the two values is greater than a threshold value S,then the transients associated with base adaptation are not yet over anda start is made again with step s7.2 via a step s7.7. The loop of stepss7.2 to s7.6 is run through so long until the difference GAG-GAGm hasbecome less than the threshold value S. In other words, the loop is runthrough until the base adaptation transients are over. The subsequentstep s7.8 operates to continuously open the tank-venting valve TEV whenbase adaptation has been stopped.

After this sequence, the base adaptation is carried out only once duringa drive cycle and thereafter the adsorption filter is permanently purgedfor opened TEV.

An additional interrupt condition is checked in step s7.7. According tothis step and after a maximum base-adaptation time span TGmax haselapsed, an opening of the tank-venting valve likewise occurs. Thisfunction assures that also for defective base adaptation, in each case,the TEV is opened.

In this case, a still further base-adaptation phase follows after atank-venting phase (approximately 1 min).

We claim:
 1. An arrangement for tank venting in an internal combustionengine, the arrangement comprising:a tank-venting assembly including atank; storage means for storing fuel vapors generated in said tank;conduit means for conducting said vapors to and away from said storagemeans; and, a tank-venting valve mounted in said conduit means foropening and closing said conduit means; an apparatus for controlling thefuel/air ratio lambda; the apparatus including: a lambda probe forsupplying a signal; a controller for receiving said signal and forforming a control output (FR) for influencing the fuel/air mixture to besupplied to said engine; means for providing a correction to adaptivelyinfluence the composition of the fuel/air mixture; and, switching meansfor switching between a first phase wherein said tank-venting valve isopened for venting said tank and a second phase wherein saidtank-venting valve is closed and the base adaptation is carried out;and, said switching means being adapted to determine the time durationof said phases in dependence upon characteristic variables of at leastone adaptive correction or upon characteristic variables of saidtank-venting assembly.
 2. The arrangement of claim 1, wherein saidcharacteristic variables of said at least one adaptive correctioninclude base adaptation (GA) and tank-venting adaptation (TEA).
 3. Thearrangement of claim 1, wherein one of said characteristic variables ofsaid tank-venting assembly is the pressure difference (Dp) between thetank pressure and the ambient pressure.
 4. A method for tank venting inan internal combustion engine having a tank-venting arrangement whichincludes a tank and a tank-venting assembly having a tank-venting valvewhich can be opened and closed for venting the tank, the engine furtherincluding a control arrangement for controlling the fuel/air ratiolambda, the control arrangement including a lambda probe for supplying alambda signal and a controller for receiving the lambda signal and forforming a control output FR for influencing the fuel/air mixturesupplied to the engine; the control arrangement further including meansfor providing a correction in the form of a base adaptation variable(GAG) to adaptively influence the composition of said fuel/air mixture,the method comprising the steps of:switching between a first phasewherein said tank-venting valve is opened for venting said tank and asecond phase wherein said tank-venting valve is closed and the baseadaptation is carried out; and, selecting the time duration of saidfirst and second phases in dependence upon at least one of the followingvariables: base adaptation variable (GAG); an additional adaptation(FTEA) carried out while said tank-venting valve is open; and, operatingdata of said tank-venting assembly.
 5. The method of claim 4, furthercomprising the step of: at full load without lambda control,continuously venting the tank with said tank-venting valve completelyopened.
 6. The method of claim 4, further comprising the step of: whenthe tank-venting valve is closed for diagnostic purposes during atank-venting phase, immediately starting a base-adaptation phase for thelambda control and extending the next tank-venting phase.
 7. The methodof claim 4, further comprising the steps of: after the start of theengine,carrying cut the base-adaptation phase until this base adaptationhas reached steady state; and thereafter, opening the tank-ventingvalve.
 8. The method of claim 4, further comprising the stepsof:monitoring the change of the base-adaptation value during thebase-adaptation phase; and, determining the base adaptation to havereached steady state when the change of the base-adaptation value dropsbelow a threshold value.
 9. The method of claim 4, further comprisingthe steps of:measuring a variable to obtain a reference quantity for thequantity of fuel to be regenerated during tank venting; and, increasingthe ratio of the time spans with and without tank venting in favor ofthe time span with venting relative to a base ratio when the fuelquantity in accordance with said reference quantity exceeds an upperlimit.
 10. The method of claim 9, further comprising the step oflowering said ratio lambda relative to said base ratio when the fuelquantity in accordance with said reference quantity drops below a lowerlimit.
 11. The method of claim 10, further comprising the stepsof:reducing the time span without venting (T₋₋ GA) only up to a pregivenminimum value when said ratio lambda is increased; and, causing saidratio lambda to increase further by extending the tank-venting time span(T₋₋ TEA).
 12. The method of claim 11, wherein the variable measured isthe pressure difference (Dp) between the tank pressure and the ambientpressure.
 13. The method of claim 11, wherein the variable measured isthe tank-venting adaptation factor (FTEA).
 14. The method of claim 11,further comprising the step of extending the tank-venting time span (T₋₋TEA) more at higher load than at lower load.